Phototropic glasses are at the present time used predominantly as spectacle sunglasses and are also increasingly finding popularity in corrective prescription lenses for defective eyesight. For correction of defective eyesight with glass having a customary refractive index of 1.523, the spectacle lenses become progressively thicker and heavier at higher diopters (more serious sight defects). For this reason, highly refractive, low-density spectacle lenses have been developed which are sufficiently light in weight to be not unpleasant to wear, even in the case of relatively large optical corrections.
Initially, it became possible to combine the criteria of phototropy, low density, low dispersion and high refractive index in a phototropic glass (German Auslegeschrift 3,117,000) in which the phototropy glass has optical properties n.sub.D equal to or greater than 1.59, an Abbe number equal to or greater than 40 and a density equal to or less than 3.2 g/cm.sup.3.
An improved low-density, lightweight phototropic glass is described in German Patent Specification 3,206,958. In order to achieve suitable phototropy kinetics, i.e., a rapid darkening when exposed to solar radiation and lightening in the absence of such exposure, these glasses have a relatively high halide content. In general, a halide content on the order of approximately 1.5%, calculated as halogen, is needed in order to achieve useful phototropic kinetics after tempering. When these glasses containing a halide component, i.e., chlorine, bromine, etc., are melted, substantial amounts of the halide component vaporize in a somewhat irregular manner, resulting in a nonhomogeneous glass. The vaporization of the halide component is especially pronounced when the glass contains lead oxide and/or zinc oxide. Consequently, in the production of such glass, it is difficult due to uneven halide vaporization to produce a homogenous glass having uniform properties.